Cytoplasm. Non-membrane and membrane organelles: endoplasmic reticulum, ribosomes, Golgi complex
The cytoplasm is the semi-liquid internal environment of the cell; it holds the organelles, inclusions and the cytoskeleton, and the main metabolic processes take place in it. Organelles are non-membrane (ribosome, cell centre) or membrane-bound (endoplasmic reticulum, Golgi complex, mitochondria, plastids, lysosomes). The endoplasmic reticulum (ER) is a system of channels and sacs connected to the nuclear envelope; smooth ER takes part in the synthesis of lipids and carbohydrates, rough ER (with ribosomes on it) in protein synthesis and transport. A ribosome consists of a large and a small subunit, contains about equal amounts of protein and rRNA, and makes proteins; its subunits are formed in the nucleolus; often several ribosomes move one after another along a single RNA strand, each making a copy of the same protein (polyribosome). The Golgi complex is a stack of flattened sacs and vesicles: it modifies the proteins and lipids made in the ER, “packages” them and directs them to where they are needed, and also takes part in renewing the membrane.
“Path of a protein”. A group shows the path of a protein as a human chain: nucleus (DNA instruction) → ribosome → rough ER → Golgi complex → vesicle → out through the membrane. Each “station” says its job.